This invention is applicable to both multi-stage and single-stage hydraulic cylinders useful in a wide variety of environments. An environment found particularly useful, and in which hydraulic cylinders are widely used, is the trucking industry where dump bodies must be raised and lowered.
A long recognized problem associated with both single-stage and multi-stage hydraulic cylinders is the accumulation of air or gas within the cylinder. The presence of this trapped air or gas is undesirable because the entrapped air will, when compressed under load, sometimes result in a somewhat erratic and thus undesirable operation. Heretofore it has been conventional to employ bleeders or channels extending from the highest entrapment point in the hydraulic cylinder to remove or "bleed" the accumulated gas or air from the cylinder. While these "bleed" systems have generally accomplished their intended result, they tend to be somewhat messy and time consuming to use.
One of these known bleeder systems, generally applicable to both single stage and multi-stage hydraulic cylinders, provides a small hole in the top of the final or smallest stage to which the upper pivot assembly is welded. A screw-in type plug is then typically located in the hole just below the upper pivot shaft. The purpose of the hole and screw-in plug is to allow the trapped air in the top of the cylinder to escape when the plug is loosened. This, in turn, allows the hydraulic fluid to enter the line or reach the hole. Then, tightening the plug only when the hydraulic fluid is present at the top of the cylinder or slightly spills therefrom, ensures that undesired air has been fully "bled" from the cylinder. This "bleeding" process must be done with the multi-stage cylinder partially extended, and can be a difficult and messy experience. Furthermore, this known "bleeding" process must be done repeatedly, virtually each time any significant amount of air is allowed to enter the cylinder from any one of a number of avenues, such as via less than perfect seals, faulty hydraulic lines, or through the hydraulic fluid reservoir.
Another known solution to the problem of air or gas entrapment involves attaching a small pressure hose with a control valve therein to the aforesaid bleed hole located at the top of the cylinder. The pressure hose connected to the bleed hole is then run down the front of the cylinder to a more convenient and accessible location, thereby allowing the bleeding process to be carried out without having to unscrew a plug at the top of the cylinder. The problems of spillage and the need for frequent bleeding are still presented.
FIGS. 1-2 exemplify a typical prior art multistage telescopic hydraulic cylinder which includes a type of bleeder system as described above. The multi-stages are in compressed or retracted form as shown in FIG. 1. The hydraulic cylinder includes outer cylinder 1 fixedly attached to base plate 21, inner cylinder 3, first intermediate cylinder 5, and second intermediate cylinder 7. Cylinders of this type are typically mounted in linkage frames, or directly to truck chassis and dump truck bodies.
Such telescopic hydraulic cylinders are usually, but not always, mounted with largest stage i fixedly mounted to base plate 21 and with smallest stage 3 axially extendable relative thereto. It is, of course, possible to invert the cylinder shown in FIGS. 1-2 so that the inner or smallest stage cylinder is fixedly mounted to a base plate and the outer or larger cylinder is axially moveable with respect thereto. In many instances, however, it is the top end of inner cylinder 3 which is typically pivotally attached to the load to be moved via pivot member 15.
In use, hydraulic fluid is pumped into base port attachment tube 25 and through base plate 21 via entrance orifice 23 using conventional hydraulic control mechanisms (e.g. power take-off device, spool valve, pumps, reservoir, and control valve systems). By pumping hydraulic fluid into and filling cylinder interior 9, largest moving stage 5 is caused by the pressure of the hydraulic fluid, to extend upward. As hydraulic fluid continues to be pumped via entrance orifice 23 into cylinder interior 9, first moving stage 5 will reach a stop at the end of its stroke, and the next moving stage (intermediate cylinder 7) will then begin to extend upward. Upon intermediate stage 7 reaching a stop at the end of its stroke, the last moving stage (inner cylinder 3) will move upward thereby extending the vertical position of pivot attachment 15 affixed to cylinder cap 17 so as to move the load attached to the cylinder to its uppermost limit. The hydraulic cylinder retracts in reverse stage order, with inner cylinder 3 first descending, then intermediate cylinder 7, and so on as fluid is forced from interior 9 of the cylinder by weight of the load via orifice 23.
Because of air that has become entrapped at the end 29 of cylinder interior 9, fluid level 27 is only permitted to reach a height dictated by the amount of air (or gas) so entrapped in the cylinder. It is well-known in the art that the trapped air in end pocket 29 interferes with the proper performance of both multi-stage and single stage cylinders. It is generally believed that this is due to the air being compressible and the fluid being relatively incompressible, causing the cylinder and thus the load to jerk or bounce as it is elevated during cylinder expansion.
The "bleeding" process for purging the trapped air from the prior art cylinder shown in FIG. 1 is accomplished via bleed-hole 11 disposed in the cylinder cap 17 and its corresponding screw plug 13. The bleeding is carried out by removing screw plug 13 from bleed-hole 11 and allowing the trapped air in the cylinder to escape upwardly through bleed-hole 11. After all of the air has escaped from the cylinder, screw plug 13 is reinserted into bleed-hole 11, but only when hydraulic fluid is present at the top of the cylinder adjacent bleed-hole 11 (or slightly overflowed) thereby ensuring that substantially all air within the confines of the cylinder has been removed. This process must, of course, be carried out with the cylinder partially extended thereby making the process both inconvenient and time consuming. Any spillage of fluid creates a clean-up problem.
FIGS. 3(a) and 3(b) illustrate prior art base plate 21 of the multi-stage cylinder shown in FIGS. 1-2. Base plate 21 defines entrance orifice 23 in a non-concentric position with respect to the base plate's outer periphery. Entrance orifice 23 acts as a hydraulic fluid passageway between interior 9 of the hydraulic cylinder and the hydraulic fluid reservoir (not shown). The hydraulic fluid, upon being pumped toward the cylinder from the reservoir, flows through orifice 23 in base plate 21 and into interior 9 of the prior art hydraulic cylinder of FIGS. 1-2.
Entrance orifice 23 is disposed at a non-central position due to the presence of pivot 30 attached to the bottom or exterior side of base plate 21. Pivot 30 is attached to the base plate at a central area thereof exterior the cylinder, rendering it difficult for the fluid to enter base plate 21 at the central area occupied by pivot 30. Accordingly, the hydraulic fluid enters the base plate and cylinder at the non-central location defined by orifice 23. Beveled portion 28 of the base plate accommodates the lower end of outer cylinder 1. Conventional pivot 30 is not shown in FIG. 3(b) for the purpose of simplicity.
U.S. Pat. No. 3,496,838 discloses a self-purging or self-bleeding hydraulic cylinder including a piston enclosed therein. In this patent, a flanged tubular purging element is coaxially secured to the base of the piston inside the cylinder. The flanged portion bears on a cup-type gasket and sandwiches the gasket between the piston base and flange, and a hollow tubular portion of the purging member depends axially downward from the flanged portion. The flanged portion has a diameter substantially less than the base of the piston. The flanged portion is radially penetrated by a plurality of radially extending orifices which communicate with the axial opening of the hollow tubular depending portion, thereby allowing compressed air to be bled from the cylinder via the orifices in the flange and the axial opening in the hollow tubular portion during contraction of the cylinder.
Further examples of known bleed systems are found in U.S. Pat. Nos. 2,588,285; 3,496,838; and 5,191,828. The bleed system of U.S. Pat. No. 2,588,285 is adapted to be used in conjunction with a single stage hydraulic cylinder including a fixed tube disposed therein. This system is fairly complex in that it requires an additional bleed tube disposed in the annular space between the inner and outer cylinders.
Aforesaid U.S. Pat. No. 3,496,838 requires the presence of a piston within the hydraulic cylinder. This is undesirable due to the fact that many commercial hydraulic cylinders avoid the use of pistons disposed therein.
U.S. Pat. No. 5,191,828 discloses a single stage hydraulic cylinder having a bleeding system therein, the bleeding system being of a highly complex nature. The large number of moving parts of this design renders it difficult to manufacture.
It is apparent from the above that there exists a need in the art for a simple, cleaner-to-operate, and inexpensive self-bleeding mechanism adaptable to use in both multi-stage and single-stage hydraulic cylinders, wherein the air trapped within the interior of the cylinder is automatically bled or purged therefrom each time the cylinder is cycled.